This invention involves the known photocatalytic effect of certain metal oxides such as titanium dioxide. More particularly, it involves compositions for coating the photocatalytic metal oxides onto carrier surfaces, thereby permitting practical means for bringing liquids or gases to be treated into contact with the catalytic material.
The use of metal oxides such as titanium dioxide to photocatalytically decompose organic material has been extensively treated in the prior art. In addition to titanium dioxide, the photocatalytic effect has been reported to have been achieved with the oxides of zinc, tungsten, and tin.
The utility of the photocatalytic compounds depends largely upon providing a solid carrier with large surface area so as to increase the contact area with the liquid or gas being treated. In U.S. Pat. No. 5,045,288 a layer of catalyst particles is loosely supported on a filter or in a granular bed. A more practical approach is to coat a solid support member with the catalyst. For example, use of porous ceramic substrates to support titanium dioxide coatings is disclosed in U.S. Pat. No. 5,035,784. Because the photocatalytic effect requires exposure of the catalyst to ultraviolet radiation, the use of a transparent substrate such as glass has been suggested. In particular, it has been recognized in U.S. Pat. Nos. 4,892,712; 4,966,759; and 5,032,241 (all to Robinson et al.) that fiberglass combines both transparency and high surface area, whereby carriers made from matrices of fiberglass are ideally suited for this purpose. Moreover, the art of fabricating high surface area materials from fiber glass (e.g., mats, filters, and fabrics--both woven and non-woven) is highly advanced and versatile. It would be highly advantageous for the sake of efficiency and cost to be able to apply a photocatalytic coating to fiber glass filaments and/or strands as they are being formed, prior to being processed into more complex product configurations. In other words, it would be desirable to apply these coatings in the same manner as sizing compositions are conventionally applied to fiber glass during its primary manufacturing stage. However, to be compatible with fiber glass sizing operations, the coating compositions need to be water based. This presents a problem because the organo-metallic compounds recommended by the prior art for photocatalytic coatings tend to be unstable in aqueous solutions due to premature gelation in the presence of water due to hydration which leads to polymerization. Providing metal oxide photocatalytic compounds that are stable in an aqueous environment would be useful in overcoming this problem.
Organic titanates available from DuPont under the name TYZOR are tetra alkoxy titanium compounds would be suitable for forming photocatalytic coatings but for their tendency to rapidly undergo hydrolysis in the presence of water. Because the suggested methods for making aqueous solutions of these organic titanates involve the use of large amounts of acid or organic solvent, these methods are unsuitable for large-scale industrial applications such as coating of fiber glass. The TYZOR line of titanates also includes chelated versions that provide some stability in aqueous media. Monofunctional ligands displace a portion of the alkoxy groups of the titanates in preparing these commercially available chelates. The chelated compounds include acetoacetates, lactates, and chelates of triethanolamine. Although useful in providing aqueous stability, these titanate chelates have been found to fall short in their film forming ability, apparently due to relatively low molecular weight increase when polymerized.
The prior art suggests the use of sol-gel techniques to apply metal oxides onto formed substrates such as woven fiber glass media (e.g., U.S. Pat. No. 4,966,759). It has been observed that the sol-gel approach results in rather thick layers of metal oxide that tend to be friable and poorly adherent to fiber glass substrates. It would be desirable to provide a durable, adherent metal oxide coatings on fiber glass from aqueous coating compositions.